Piston pump having cylinder with leak opening

ABSTRACT

A high pressure/vacuum piston pump has a reciprocating piston in a cylinder which has a small, preferably about 0.05 inches in diameter, leak opening allowing some of the working air to leak to the pump housing during at least a portion of the piston stroke. The leak opening is coplanar with the drive shaft and the centerline of the cylinder. It is also located to be between the piston head and the valve head for the majority of the piston stroke but below the piston cup seal and closed off from the working chamber when the piston is at top dead center. During transient off periods of the pump the leak path relieves high pressure or vacuum gas in the cylinder allowing a low-torque motor to restart the pump reliably.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/446,322 filed on Feb. 6, 2003.

STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] This invention relates to piston pumps, and more particularly tohigh pressure or vacuum pumps with improved restarting capabilities.

[0004] Certain pumping applications require the pump to operate inon-off cycles. In high pressure or vacuum applications, on the order of100 psi or more, the volume of air in working chamber(s) of the pumpcylinder(s) can impede reciprocation of the piston(s) significantly suchthat restarting the pump requires a large amount of torque. This trappedair can tax the pump drive motor excessively and cause premature motorwear, difficult, inconsistent and failed restarting, and/or requireexpensive motors having higher starting torque.

[0005] There have been many attempts to make restarting easier or moreconsistent. One technique is to use a larger motor that is significantlyoversized for normal operation, but which would provide high startingtorque sufficient to overcome the resistance of the trapped cylinderair. Another, significantly more efficient technique is to create a leakpath at the intake port (or exhaust port when pulling a vacuum) in thevalve plate of the pump. This can be done by ramping an interior surfaceof the valve plate at the intake port such that the flapper valvecontrolling flow through the port does not fully seat and close off theintake port until after a threshold cylinder pressure is reached. Thistechnique allows a relatively low-torque (and thus less expensive) drivemotor to be used. However, it requires rather expensive and timeconsuming machining of the valve plate to form the ramp. It can alsofatigue the flapper valve at the intake and significantly reduce theflow rate of the exhausted air flow, which may be unsuitable for certainapplications requiring specific or minimum flow rates.

[0006] Accordingly, a high pressure/vacuum pump with improved restartingcapabilities is needed.

SUMMARY OF THE INVENTION

[0007] The present invention provides a piston pump having a housingdefining an internal cavity in which is disposed a cylinder defining acylindrical passage in which a head of a piston reciprocates to vary thevolume of a working chamber of the cylinder. The cylinder has a leakopening providing communication between the working chamber and anambient atmospheric pressure during at least a portion of a pistonstroke.

[0008] The leak opening is located such that it is between the pistonhead and the valve head for the majority of the piston stroke.Preferably, the leak opening has a centerline lying in a planeintersecting the shaft axis and the centerline of the cylinder,preferably parallel to the shaft axis and perpendicular to the cylindercenterline. The leak opening is further located proximate to a top endof the cylinder within a spacing of less than about 0.2 inches, morepreferably its center is about 0.05 inches from the top end of thecylinder. The piston head includes a seal slideably mating with an innerdiameter of the cylinder. The center of the leak opening should belocated no more than about 0.1 inches, preferably at about 0.05 inches,below the piston cup seal when the piston is at top dead center so thatthe piston cup traverses the opening and temporarily closes it near topdead center.

[0009] The leak opening may be sized differently and may be located atmore or less distance from the top end of the cylinder for differentsized cylinders. The size and location of the leak opening is generallyindependent of the piston stroke. Generally, increasing the leak openingincreases the restart reliability and decreases the flow rate of thepump and locating the leak opening further from the top of the cylinder(and thus the piston seal at top dead center) decreases the restartreliability and increases flow rate. However, regardless of cylindersize the leak opening should be of a similar order of magnitude in sizeand should be located nearer the top end of the cylinder than the bottomend.

[0010] The pump of the present invention having the leak opening in thecylinder provides for bleeding of working gas from the cylinder to theexterior of the cylinder. The bleeding is intermittent in that thepreferred location of the leak opening is such that the piston cuppasses from below to above the leak opening at each stroke to top deadcenter. This intermittent bleeding is such that, during transient offperiods of the pump, the cylinder can be sufficiently depressurized toallow rapid restarting of the pump with a relatively low torque drivemotor. Moreover, the leak opening can be a simple through bore formed bya quick and low-cost boring operation.

[0011] The present invention thus provides a pump having high restartreliability particularly suited for high pressure or vacuum on-offcycled applications. The improved restarting capabilities of the pump isprovided by a simple boring operation without the need for expensivemachining operations. Since the leak path is located in the cylinder atthe proper location, flow rate losses are reduced as is valve fatigue.

[0012] The foregoing and other objects and advantages of the inventionwill appear from the following description. In this descriptionreference is made to the accompanying drawings which form a part hereofand in which there is shown by way of illustration a preferredembodiment of the invention. This embodiment does not necessarilyrepresent the full scope of the invention, however, and reference mustbe made therefore to the claims for interpreting the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a cut-away side view of a piston pump according to thepresent invention with a housing cover removed to show internalcomponents thereof;

[0014]FIG. 2 is a partial cross-sectional view taken along line 2-2showing a piston in a cylinder of the pump near the beginning of thestroke; and

[0015]FIG. 3 is a view similar to FIG. 2 showing the piston at top deadcenter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016]FIGS. 1 and 2 shows a single cylinder high pressure/vacuum pistonpump 10 with improved restarting capabilities according to the presentinvention. The pump 10 generally includes a housing 12 mounting a motor14 with a rotatable drive shaft 16 extending along a shaft axis 17 intoa crank chamber 18 in the housing 12. The shaft 16 mounts a fan 20 and aconnecting rod 22 of a piston 24 having an enlarged circular head 26disposed in the interior of a cylinder 28 inside the housing 12 so as tobe generally perpendicular to the shaft 16, that is the shaft axis 17 isessentially perpendicular to a centerline 29 of the cylinder 28. Apiston cup seal 30 is captured between the head 26 and a retainer 32.The piston cup seal 30 is sized to slideably seal against the innerdiameter of the cylinder 28 as the piston 24 is reciprocated up and downby rotation of the shaft 16. A valve plate 34 having suitable intake andexhaust valves (not shown) controlled by thin metal flapper valves (notshown) is mounted to a top side of the housing 12 and sealed against atop end 36 of the cylinder 28. A valve head 38 mounts onto the valveplate 34 and has intake 40 and exhaust 42 fittings for coupling intakeand exhaust lines (not shown) as needed for the desired application.

[0017] As shown in FIGS. 1-3, a leak opening 44, preferably a simplebore, extends transversely through the wall of the cylinder 28 providingcommunication between the interior of the cylinder 28 and the interiorof the housing 12. The leak opening 44 is located near the top end 36 ofthe cylinder 28 such that it is positioned between the piston 24 and thevalve plate 34 during most, in fact, nearly all, of the piston stroke asshown in FIG. 2. In a preferred location, the leak opening 44 ispositioned so its centerline 45 lies in a plane containing the shaftaxis 17 as well as the centerline 29 of the cylinder 28, being parallelto the shaft axis 17 and perpendicular to the cylinder centerline 29.The leak opening 44 is also preferably positioned so that the piston cupseal 30 is below it except for as the piston 24 nears top dead center(i.e., its highest position) as shown in FIG. 3 at which point thepiston cup 30 passes beyond the leak opening 44 and closes communicationto the working chamber. In this way, the leak opening 44 is at theworking side of the piston head 26 the majority of the time and is atthe non-working side of the piston head 26 only while at or very neartop dead center. Put another way, the leak opening 44 is above thepiston head 26 when the crank angle is approximately between 10 to 350degrees. This allows the leak opening 44 to bleed the working air out ofthe cylinder 28 during nearly the entire piston stroke. Yet, the leakingor bleeding is intermittent in that there is no leakage for the crankangles at or near top dead center.

[0018] The leak opening 44 is relatively small (compared to the volumeof the cylinder 28) so that during steady state operation of the pump 10at higher speeds, sufficient pressure or vacuum can be achieved toprovide high flow rates. Yet, when the pump 10 is de-energized oroperating at low speed like when starting up, the leak opening 44 willallow the working air in the cylinder 28 to bleed rapidly out thecylinder 28 so that the reciprocation of the piston 24 is less impeded,thereby allowing restarting with a low-torque motor. And, since the leakopening 44 is at the working side of the piston 24 for most of itsstroke, when the pump 10 is stopped the piston cup 30 will most likelybe below the leak opening 44 allowing bleeding of the compressed air inthe cylinder 28 to prepare for the next restart. Moreover, when the pump10 is stopped and piston 24 ceases being driven by the motor, theresidual working air in the cylinder 28 acts to force the piston 24 downto at or near bottom dead center. This makes it even more likely thatthe leak opening 44 would be in communication with the working chamberof the cylinder 28. Thus, the leak opening 44 will consistently providethe necessary bleeding function to ensure consistent restarting of thepump, even when using a low-torque motor.

[0019] In one preferred application, the pump 10 uses a low-torque motorproviding 8 oz.-ft of starting torque and the pump 10 providesapproximately 0.5 CFM output flow rate at about 60 psi. The cylinder 28has a 2.25 inch inner diameter and 0.125 inch wall thickness. The pistonstroke is about 0.32 inches. In this case, the leak opening 44 ispreferably no more than about 0.1 inches in diameter, more preferablyabout 0.05 inches in diameter. The leak opening 44 is preferably locatedwithin about 0.5 inches of the top end 36 of the cylinder 28, morepreferably at less than about 0.2 inches from the top end 36, and nomore than about 0.1 inches, preferably at about 0.05 inches, below thepiston cup seal 30 when the piston 24 is at top dead center. The abovedimensions are taken from the horizontal centerline of the leak opening44.

[0020] It should be noted that this is only one example of a preferredpump application. The leak opening 44 may be sized differently and maybe located at more or less distance from the top end 36 of the cylinder28 for different sized cylinders, operating pressures and/or flow raterequirements. Regardless of these parameters, the leak opening 44 shouldbe of a similar order of magnitude in size and should be located nearerthe top end of the cylinder than the opposite (bottom) end.

[0021] As mentioned, the leak opening 44 is aligned with the drive shaftaxis 17, which has been found to provide the greatest flow rate (roughly25% higher than a leak opening positioned perpendicular to the shaftaxis). It has also been determined empirically that, while increasingrestart reliability, putting the leak opening 44 lower (further awayfrom the top of the cylinder 28 and the piston cup seal 30 at top deadcenter) significantly decreases the output flow rate of the pump.Generally, the leak opening 44 location is independent of piston stroke.The diameter of the leak opening 44 can vary depending on the flowrequirements of the application, preferably keeping a minimum diameterto prevent debris from clogging the leak opening 44. Generally,increasing the diameter of the leak opening 44 will improve the restartreliability but decrease the flow rate of the pump.

[0022] An illustrative embodiment of the invention has been described indetail for the purpose of disclosing a practical, operative structurewhereby the invention may be practiced advantageously. However, theapparatus described is intended to be illustrative only, and the novelcharacteristics of the invention may be incorporated in other structuralforms without departing from the scope of the invention. For example,the precise size and location of the leak opening can vary depending onflow rate and pressure requirements of the application. Moreover, whileonly a single leak opening is described above and shown herein, it iswithin the scope of the invention for the cylinder to have multiple leakopenings at various locations in the cylinder. Additionally, while a 60psi application was been described, the improvement in restartreliability and flow rate can be realized in higher pressure or vacuumapplications of 100 psi or more. Accordingly, to apprise the public ofthe full scope of the invention, the following claims are made:

What is claimed is:
 1. In a piston pump having a housing defining aninternal cavity in which is disposed a cylinder defining a cylindricalpassage in which a head of a piston reciprocates to vary the volume of aworking chamber of the cylinder, the improvement wherein the cylinderhas a leak opening providing communication between the working chamberand an ambient atmospheric pressure during at least a portion of apiston stroke.
 2. The improvement of claim 1, wherein the leak openingis located such that it is between the piston head and the valve headfor the majority of the piston stroke.
 3. The improvement of claim 2,wherein the leak opening is located proximate to a top end of thecylinder.
 4. The improvement of claim 3, wherein the leak opening islocated at less than about 0.2 inches from the top end of the cylinder.5. The improvement of claim 4, wherein the piston head includes a cupseal slideably mating with an inner diameter of the cylinder and whereina center of the leak opening is located no more than about 0.1 inchesbelow the piston cup seal when the piston is at top dead center.
 6. Theimprovement of claim 5, wherein the center of the leak opening is about0.05 inches above the piston cup when the piston is at top dead center.7. The improvement of claim 1, wherein the leak opening is less thanabout 0.1 inches in diameter.
 8. The improvement of claim 7, wherein theleak opening is about 0.05 inches in diameter.
 9. The improvement ofclaim 1, wherein the piston is connected to a drive shaft extendingalong a shaft axis and wherein the leak opening has a centerlinedisposed in a plane containing the shaft axis and a centerline of thecylinder.
 10. The improvement of claim 9, wherein the leak openingcenterline is parallel to the shaft axis and perpendicular to thecylinder centerline.